All of the Legacies cards which indicate the "die showing damage" language include the indirect damage symbol. Are cards from earlier sets now also considered to have the indirect symbol when they indicate "a die showing damage"?

Damage is damage, those icons are just reminders. Unless there is a ruling out there I am not aware of.

Correct, the damage types shown in parentheses are ‘reminder text’ only.

Anything that refers to ‘damage’ without specifically calling out a type (eg Deflect/Dodge/Parry, etc) now includes Ranged/Melee/Indirect.

I assumed as much. If a card just says “Damage” it covers all of them. If it specifies (Ranged, Melee, Indirect) then it only handles that type. Note that a lot of the stronger defense cards (especially die removal) specify a type of damage. This is good as otherwise they would be OP.

Note that prior to the current RRG (1.6) the parenthetical icons were not clearly reminder text. 1.6 was updated to include all parenthetical text as reminder text, not just italicized text.